1. Field of the Invention
The present invention relates to a liquid crystal display having excellent brightness without moiré patterns. The present invention also relates to a prism sheet and a backlight unit which may improve the brightness of the liquid crystal display without developing the moiré patterns.
2. Description of the Related Art
The liquid crystal display, also know as LCD, is an electronic device that transforms electrical signals into visual signals by utilizing the change in the transmittance of liquid crystals according to applied voltages.
As well known in the art, the liquid crystal display is a non-emitting display device. Therefore, the liquid crystal display needs to use an outside light source unit for illuminating uniformly the viewing plane of liquid crystal panel outside the liquid crystal panel in order to display visual information. The backlight unit is conventionally used to provide light to the viewing plane of the liquid crystal panel.
The backlight unit may be classified into two types: direct type and edge-light type according to the position where the light source is disposed. In the direct type, the light source is disposed directly at the back of the liquid crystal panel, whereas the light source is disposed along a side surface of the light guide plate in the edge-light type.
FIG. 1 is a perspective view of the backlight unit for illuminating the liquid crystal panel from the back of the liquid crystal panel, wherein the backlight unit is of the edge-light type.
As shown in FIG. 1, the backlight unit 10 comprises a light source unit 1, a reflector sheet 2, a light guide plate 3, and a plurality of optical sheets 4, 5 and 6.
The light source unit 1 comprises a light source 1a and a light source reflector 1b. The light source 1a is received inside the light source reflector 1b, and disposed along the side surface of the light guide plate 3. The light source reflector 1b reflects the light generated at the light source 1a toward the light guide plate 3, thereby the light efficiency of backlight unit 10 can be improved.
The light guide plate 3 mixes the light input through the light incidence surface disposed at its side surface and emits the mixed light through the light emitting surface disposed at its upper surface in a direction toward the liquid crystal panel (not shown).
The reflector sheet 2 reflects the light emitted through the lower surface of the light guide plate 3 so that the light is re-input into the inside of the light guide plate 3, which improves the light efficiency of the backlight unit 10.
The optical sheets 4, 5 and 6 may be comprised of a diffuser sheet 4, a prism sheet 5 and a protector sheet 6. Hereinafter, the functions of each component of the optical sheets 4, 5 and 6 will be described.
The light emitted through the upper surface of the light guide plate 3 enters into the diffuser sheet 4. The diffuser sheet 4 scatters the light to make the brightness uniform and widen the viewing angle.
Because the brightness declines sharply while the light passes through the diffuser sheet 4, the prism sheet 5 is provided in the backlight unit 10 to compensate such declination of brightness. The prism sheet 5 refracts the light emitted from the diffuser sheet 4 in a low angle to collimate the light toward the front direction, thereby the brightness is improved within the effective viewing angle.
FIG. 2 is a cross-sectional view of the prism sheet of FIG. 1 taken along the line I-I′.
Referring to FIG. 2, the prism sheet 5 is comprised of a base film 10, a prism base 11 disposed on the base film and a plurality of prisms 12 disposed on the prism base.
The prism sheet 5 reflects and resends toward the light guide plate (3 of FIG. 1) a portion of the light emitting from the diffuser sheet (4 of FIG. 1) in a certain angle. The prism sheet 5 also refracts the remaining portion of the light to be collimated toward the front direction where the liquid crystal panel (not shown) is disposed, thereby the brightness is improved within the effective viewing angle.
Referring back to FIG. 1, the protector sheet 6 is disposed over the prism sheet 5. The protector sheet 6 prevents the surface of the prism sheet 5 from being damaged, and also widens the viewing angle narrowed by the prism sheet 5.
The liquid crystal panel generally includes repeated patterns of a plurality of transistors and electrode components. However, the patterns of the liquid crystal panel along with the other patterns of the optical devices provided in the backlight unit illuminating the liquid crystal panel develops moiré patterns.
Information relevant to attempts to address the problem of the moiré patterns can be found in U.S. Pat. No. 6,091,547 (3M Innovative Properties Company), which discloses a luminance control film having less than 30 μm pitched prisms. However, the luminance control film of the reference suffers from the deterioration of the brightness where the pitch of the prism is less than 30 μm. And such fine pitch is also not advantageous in terms of manufacturing process or thermal and mechanical properties.